Image quality in industrial radiography a crucial safety factor

By Heinrich Heidt, ISO/TC 138, Non-Destructive Testing, Subcom-mittee SC 5, Radiation methods

I ndustrial radiography makes an important contribution to public safety and industrial reliability: industrial plants and power stations, gas tubes and pressure equipment need inspection by Non-Destructive Testing (NDT) methods. Besides ultrasonic inspection, radiography is the only (and oldest) method for the discovery of inner flaws and defects in welds, cast-ings, aircraft and many other devices in our technical environment.

A radiograph is the projection of a specimen onto an imaging device, traditionally an X-ray film. The source

of radiation may be an X-ray tube or a radioisotope, i.e. Cobalt 60 or Iridium 192. The evaluation of the radiograph traditionally film reading is most-ly done by a human, well-trained film interpreter.

The detectability of small defects, i.e. cracks, depends on a suf-ficient image quality of the radiograph. Good image quality means parameters for unsharpness, contrast and graini-ness which are adapted to the capabil-ities of the human eye, eventually sup-ported by a magnifying glass. The radi-ographer can influence the image qual-ity in a wide range by the selection of tube voltage, geometric distances, film type, exposure time, etc.

How can image quality be checked ?

The interpreter has to check whether the radiographer carried out an adequate job and whether the radio-graphic image has the full information content required for safe interpretation; if it does not, the radiograph must not be used since the result of interpreta-tion will be incomplete and possibly dangerous.

Main FocusWire type IQI, ISO IQI will be equal to EN IQI except for the denomination ISO

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About the author

Dr. Heinrich Heidt, Chair of SO/TC 135/SC 5,

Radiation Methods, and ead of depart-

ment at the Federal Institute or Materials

Research and Testing (BAM),

Berlin, Germany, has been involved in thestandardization of Non-Destructive Testing (NDT) methods for more than 20 years. As Convenor of German, European and interna-tional committees, he has contributed essen-tially to a uniform, worldwide application of radiographic inspection methods. A number of trade barriers and superfluous expensive multiple inspections of objects have been removed by ISO/TC 35s standards. E-mail : heinrich.heidt@bam.de

It is important to prove the image quality by a method which is independent from the specimen under inspection. For about 70 years, two basic types of Image Quality Indica-tors (IQI) are in use : the wire type IQI and the step-hole type IQI. Both are placed on the source side of the speci-men, and are imaged on the radiograph as reference objects. Both are sensitive to contrast and, in part, to unsharpness. Because of the low cost, these reusa-ble IQIs are used for nearly 100 % of radiographs worldwide.

From national to International Standards

About 20 years ago, we had manifold national standards for wire type and step-hole type IQI (Image Quality Indicators). There were differ-ences in diameter, in number of wires per IQI, in the form of plaquettes for step-hole type IQI, etc. In interna-tional trade, some goods were reject-ed because of wrong IQIs on the accompanying radiographs ! To avoid this situation, some radiographers used several IQIs on the same film, causing additional cost and covering up useful information on the radiograph.

This became, then, an urgent problem, in Europe to begin with. Within the European Committee for Standardization, CEN/TC 138, WG 1 started with an investigation of which IQIs were most frequently used, and which IQIs were redundant. Finally, one design for wire IQI and one for step-hole type IQI were selected and

standardized in EN 462 parts 1 and 2 (1994).

In 1992, the outdated, very imprecise and hardly applicable ISO 1027:1983 was due for revision or withdrawal. Since the series of standards under EN 462 had mean-while been developed to a generally acknowledged system of five parts, including an originally British IQI for unsharpness evaluation, ISO/135/SC 5 proposed to adapt the CEN Standard instead of making a revision of ISO 1027.

After a 100 % approval from the ISO member bodies via the Fast Track Procedure, the new ISO 19232, Non-destructive testing Image quality of radiographs Part 1 : Image quality indicators (wire type) Determination of image quality value, appeared in July 2004, and will be applied world-wide. ASTM has similar IQIs under E 747-97 and E 1025-98 with a differ-ent design.

Radiographic weld inspection for a pressure vessel.

Radiograph of a welded tube.

Regular versus poor, grainy image of a weld.

erwise, the appropriate IQI number requirement can be determined by an experiment carried out according to ISO 19232 part 4 under well defined exposure conditions.

If the requirement is fulfilled, the film interpretation is safe and may proceed ; the radiograph has the tech-nically achievable information content. Otherwise the exposure must be repeated under improved conditions.

Step-hole type IQI

The radiographic image has to have the full infor-mation content required for safe interpretation.

Reading the Image Quality Indicators

When the film interpreter rec-ognizes a certain smallest wire or hole of the IQI, he will protocol the number of this detail and compare it to the minimum required IQI number. This required IQI number can be tak-en from ISO 19232 part 3, if the speci-men is made of ferrous materials. Oth-

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From X-ray film to electronics

In the 1990s, radiography start-ed a fast change from film radiography to electronic imaging devices. Nowa-days there are radioscopic real time sys-tems, different types of imaging plate systems with reusable storage phosphor screens, high resolution film digitizers, and several kinds of direct and indirect converting digital detector arrays (so-called flat panels). All give access to image enhancement and quantitative evaluation, some are capable of auto-matic image interpretation.

The simple IQIs of ISO 19232 were developed for film radiog-raphy and cover only a part of the image quality parameters and control settings of electronic systems. For the analysis of the requirements of the users of elec-tronic imaging devices and the prepa-ration of additional standards, an inter-national Task Group Digital Industrial Radiology was founded by the Inter-national Institute of Welding (IIW), Sub-Commission VA Radiography-based Weld Inspection Topics . Experts are still welcome to join this work !

Standards have to reflect a chang-ing state of the art , and their applica-tion must be effective, efficient and gen-erally accepted. For industrial radiogra-phy, the quality of the image is crucial for the safety of the inspected specimen. With the inexpensive Image Quality Indicators described in ISO 19232, each radiographer and film interpreter has a reliable tool to check the basic proper-ties of the radiograph.

Main Focus

Real time radioscopic system for wheel inspection.

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